Mistletoe lectin I forms a double trefoil structure

The quaternary structure of mistletoe lectin I (MLI), a type II ribosome inactivating protein, has been determined by X-ray crystallography. A definitive molecular replacement solution was determined for MLI using the co-ordinates of the homologue ricin as a search model. MLI exists as an [AB] 2 dimer with internal crystallographic two-fold symmetry. Domain I of the B chains is non-covalently associated through interactions involving three looped chains (α, β, γ) in each molecule of the dimer, forming a double trefoil structure. The ricin molecule which shares 52% sequence homology with MLI has a disulphide bridge between Cys 20 and Cys 39 in the α loop. An evolutionary mutation has replaced Cys 39 with serine in MLI. This mutation appears to allow the α loop the flexibility required to take up its place at the dimer interface, and also suggests a rationale for why ricin does not form dimers. Measurement of retention times using FPLC gel filtration confirms that dimerisation also occurs in solution between MLI B chains with an association constant K a=10 6 M.